Wire grid polarizers are widely used in, for example, devices for graphic information imaging (e.g., see U.S. Pat. No. 6,452,724, incorporated herein by reference). The commonly-used technology for manufacturing these devices is based on optical or interference lithography. However, the cost associated with the use of the tools designed for these applications is considered very significant. The existing approach and tools make it difficult to scale the production from smaller semiconductor wafer sizes to larger area substrates (such as glass or plastic sheets). In addition, the existing approach makes it is very difficult to create wire grid structures with a period of 150 nm or less. While different applications have different requirements, structures with smaller feature size are usually associated with higher performance.
A method for nanorelief formation on a film surface, utilizing plasma modification of a wave-ordered structure (WOS) formed on amorphous silicon layer, was disclosed in Russian Patent Application RU 2204179, incorporated herein by reference.
An example of this approach is schematically illustrated on FIGS. 1A and 1B. First, a layer of amorphous silicon 2 is deposited on top of the target thin film layer 4. Then, the silicon layer is sputtered with a flow of nitrogen ions 31 to create a conventional wave ordered nanostructure 1. The resultant wave-ordered nanostructure has relatively thick regions of amorphous silicon nitride 10 and relatively thin regions of amorphous silicon nitride 20 situated respectively on the front and back sides of the waves in the wave-ordered structure 1. As shown, the wave troughs are spaced from the surface of the film layer 4 by a distance D that is usually less than the nanostructure period 3. After the wave-ordered nanostructure 1 is formed, its planar pattern, which is shown in FIG. 1A, is transferred into the underlying film layer 4 by selectively etching the amorphous silicon layer 2 while using regions 10 and 20 as a nanomask.
However, experiments using conventional wave ordered nanostructures obtained by single-step oblique sputtering of amorphous silicon with nitrogen ions showed that these structures often do not possess a desired degree of ordering (i.e., high coherency). FIGS. 2A and 2B show an array of nanostructures 21 manufactured by this technique. The array is composed from amorphous silicon nanostripes 2 covered by the regions of amorphous silicon nitride 10. The nanostripes are separated by trenches 22. FIG. 2A shows that even in a relatively small area this array has a significant number of defects: bends, connections, and breaks. It may not be sufficiently coherent enough for optoelectronic applications.
A coherent hard nanomask and methods of its formation are described in U.S. Pat. No. 7,768,018 and U.S. Patent Application Publication No. 2006/0273067 and methods of formation of coherent wavy nanostructures are disclosed in U.S. Pat. No. 7,977,252 and U.S. Patent Application Publication No. 2008/0119034, all of which are incorporated herein by reference. However, in those methods the highest degree of the nanomask ordering is provided by oriented surface polishing in the first direction prior irradiating the surface with an ion beam. The step of oriented surface polishing has a contact nature, while it is preferable to use contactless methods to improve manufacturability and scaling of the nanomask over large areas.
A contactless method for improving the ordering of nanoscale ripple patterns is described in Adrian Keller and Stefan Facsko, Tuning the quality of nanoscale ripple patterns by sequential ion-beam sputtering, Physical Review B, Vol. 82, pp. 155444-(1-8), 2010, which is incorporated herein by reference. In this method, at the first step, the surface of monocrystalline silicon is obliquely irradiated with argon ions in a first plane of ion incidence to form a ripple pattern with ripples mostly elongated in a first direction which is perpendicular to the first plane. At the second step, the ripple pattern is irradiated at grazing angle with argon ions in a second plane of ion incidence which is perpendicular to the first plane and parallel to the first direction. The second step of ion irradiation results in improvement of ordering of the ripples formed at the first step due to 40% reduction in the density of the ripples' connections and breaks. However, in this method, the resultant ripples can hardly be used as a nanomask because, at the second step, most ripples are irradiated by ions from both sides simultaneously and symmetrically and, hence, both sides of such ripples are modified by ions equally. In addition, there is no considerable difference between the initial and the resultant ripple pattern quality and the attainable degree of the resultant ripple pattern ordering may not be sufficient for optoelectronic applications.